Most artificial satellites, spacecraft and other propelled devices, such as aircraft, ship and ground vehicles (collectively referred to herein as vehicles), require information about their locations and/or attitudes to accomplish their missions. This information may be obtained from one or more sources, such as an inertial navigation system (INS), a global positioning system (GPS), ground-based radar tracking stations and/or an on-board star tracker.
A star tracker is an optical device that measures bearing(s) to one or more stars, as viewed from a vehicle. A star tracker typically includes a star catalog that lists bright navigational stars and information about their locations in the sky, sufficient for a processor to calculate a location of a vehicle in space, given bearings to several of the stars. A conventional star tracker includes a lens that projects an image of a star onto a photocell, or that projects an image of one or more stars onto a light-sensitive sensor array (digital camera).
One type of star tracker is “strapped-down,” meaning its view angle, relative to its vehicle, is fixed. Another type of star tracker can be aimed mechanically, such as in a direction in which a navigational star is expected to be seen. Using data from the photocell or sensor array, the star catalog and information about the star tracker's view angle, relative to the vehicle, a processor in the star tracker calculates a position of the vehicle in space.
Strapped-down star trackers are mechanically simpler than mechanically aimable star trackers. However, the fixed view angle of a strapped-down star tracker limits the number of navigational stars that may be used. Mechanically aimable start trackers can use a larger number of navigational stars. However, aiming a prior art star tracker, relative to its vehicle, with the required precision poses substantial problems. In either case, preventing stray light, such as from the sun or reflected from the moon, reaching the photocell or sensor array is challenging, particularly when a navigational star of interest is apparently close to one of these very bright objects.
Conventional strapped-down and mechanically aimable star trackers are large, heavy and consume a large amount of energy. For example, a large lens is massive, and its focal length distance between the lens and sensor contribute to the volume occupied by a star tracker. Smaller and lighter star trackers are desirable.
A Butler matrix (first described by Jesse Butler and Ralph Lowe in “Beam-Forming Matrix Simplifies Design of Electronically Scanned Antennas,” Electronic Design, volume 9, pp. 170-173, Apr. 12, 1961, the entire contents of which are hereby incorporated by reference herein for all purposes) is a type of passive phasing network having N inputs and N outputs, usually a power of two. A Butler matrix, coupled between a set of antenna elements and a transmitter or a receiver, may be used for beamforming.
The N inputs of a Butler matrix are isolated from each other. Phases of the N outputs are linear, with respect to position, so the beam is tilted off the main axis. None of the inputs provides a broadside beam. The phase increments, among the outputs, depend on which input is used. For example, a Butler matrix may be constructed such that when input port 1 is used, the four outputs are linearly phased in 45 degree increments; when input port 2 is used, the four outputs are linearly phased in 135 degree increments; when input port 3 is used, the four outputs are linearly phased in 270 degree increments; and when input port 4 is used, the four outputs are linearly phased in 315 degree increments. Thus, depending on which of the N inputs is accessed, the antenna beam is steered in a specific direction in one plane. Two Butler matrices can be combined to facilitate 3-dimensional scanning.